As to heat-resistant polymers, there have been studied fluorine-containing materials and aromatic or heterocyclic polymers such as polybenzimidazoles, aromatic polyamides and polyimides since synthesis of silicones by Rocho, et. al. Further, in recent years, with the development of technology in the space and aviation fields, demand for heat resistance has been increased, and the study has been advanced with respect to aromatic or heterocycyclic high polymers, with aromatic polyimides being under improvement. In addition to them, organometallic polymers having a main chain comprising metal element such as Si, Ti or B and O, N, etc. such as polyborosiloxane and polytitanosiloxane have been studied.
Also, in recent years, further improvement in heat resistance has been required with respect to heat-resistant and insulating materials to be used in various uses. For example, as one example of the heat-resistant and insulating materials, there is illustrated the field of high-pressure rotary machines such as generators, high-pressure alternating current motors, industrial direct current motors and motors for electric trains. These high-pressure rotary machines tend to come to be large-capacity-oriented and high-pressure-oriented and, at the same time, have been required to reduce size and acquire high heat resistance. Some of the heat-resistant and insulating materials in these fields are required to have a heat resistance of higher than 400° C. in the atmosphere and, at the same time, high insulating properties, high flexibility and high mechanical properties.
As other uses for the heat-resistant and insulating materials, there are illustrated various uses including an insulating or dielectric layer for displays such as PDP and LCD, an interlayer dielectric film for a thin film semiconductor device, materials for a semiconductor element such as Pre-Metal-Dielectric Layer (PMD) adapted for a 600° C. low-temperature process, front panels for displays, heat-resistant films, heat-resistant sheets or heat-resistant coatings for use in various uses, plastic optical fibers, heat-resistant and insulating threads, and electric wire coating. In these uses, some materials are required to have optical and electric properties such as transparency and dielectric properties in addition to the above-described heat resistance, flexibility and mechanical properties. Further, additional uses thereof in the field of ceramics composite materials, ceramic-metal adhesives, high-performance products with materials such as ceramic materials such as Al2O3 or MgO, or high-performance products obtained by blending with other polymer have been tried as well. However, as to organic polymers, heat-resistant polymers at present available have a practical heat-resistant temperature as low as 300° C. or less.
On the other hand, organometallic polymers such as silicone resins and polyborosiloxane resins have a high heat resistance as high as 500° C. or above due to thermal stability of the polymers. However, while the organic polymers have a molecular structure of a linear structure with less branching, the organometallic polymers have a network structure with more branching wherein a huge three-dimensional network structure is formed by dehydrogenation reaction or oxidation reaction between OH groups and, thus, they have such a poor flexibility that their uses have been limited.
In order to solve the defect with the organometallic polymers, it has been attempted to make a composite material between an organic resin and an inorganic material. For example, a block copoly-silazane composed of an inorganic polysilazane moiety and an organic polysilazane moiety has been proposed (for example, JP-A-2-175726). However, such composite polymer yet has a problem of, for example, insufficient mechanical properties, thus still not being satisfactory.
With such circumstances in mind, the inventor has formerly developed and proposed a highly heat-resistant, silicon-containing copolymer comprising silazane copolymer containing a specific repeating unit (JP-A-8-231727 and JP-A-9-188765). This silicon-containing copolymer is an organometallic polymer having a heat resistance of 400° C. or higher, an excellent mechanical properties and an excellent flexibility, which can be produced with ease and has the property of becoming transparent upon being cured in a nitrogen gas. Therefore, it is a useful material for highly heat-resistant optical use. However, when cured in the atmosphere, there results a colored polymer, thus there arises a problem that a film with a good transparency is difficult to obtain. Accordingly, the silicon-containing copolymer requires to be cured in a nitrogen gas in order to obtain a transparent cured polymer, which sometimes makes it difficult to apply the polymer to actual uses in view of process cost and mass production.
As a result of investigation, the inventor has formerly developed a silicon-containing copolymer which does not involve the problem of coloration upon curing by heating in the atmosphere, which has a heat resistance of 400° C. or higher, which has an excellent mechanical strength and an excellent flexibility, and which has a high transparency and a low relative dielectric constant, and filed an application on the polymer as Japanese Application No. 2001-99092 (JP-A-2002-293941, WO/02/081553). The silicon-containing copolymer proposed in this Japanese Patent Application No. 2001-99092 is transparent and has an excellent heat resistance and a low dielectric constant, and hence it might find application as flat panel displays (FDP) as well as conventional application to formation of heat-resistant films, hard coating films or insulating films. PDP having a baked film of the silicon-containing copolymer proposed in Japanese Patent Application No. 2001-99092 as a dielectric layer for covering a transparent electrode or an address electrode has been filed as Japanese Patent Application No. 2001-108593 (JP-A-2002-304949). However, the silicon-containing copolymer has a curing temperature of as high as about 400° C. In the case of using the cured silicon-containing copolymer as, for example, a material for various film-forming materials for LCD or PDP or a sealing material, the curing temperature is in many cases required to be 250° C. or lower in the process for producing LCD or PDP, thus the aforesaid polymer which requires curing at a temperature of 350° C. or higher not being usable in some cases. If the curing temperature of the polymer can be decreased to 250° C. or lower, it can find application to most of FPD uses such as LCD and PDP, thus serving to acquire an increased utility. Also, the polymer is required to be improved with respect to chemical resistance such as alkali resistance. Further, heat resistance, mechanical properties, flexibility, transparency and relative dielectric constant of the polymer are required to be more improved in comparison with those of conventional polymers. Also, materials which allow to form a coat having a more thickness by one coating are required. Further, in forming a dielectric layer or the like for PDP, a process of heating at a high temperature under a reduced pressure is unavoidable, and thus the silicon-containing copolymer proposed in Japanese Patent Application No. 2001-99092 is required to have a more improved durability against high-temperature heating under reduced pressure so that, when heated to a high temperature under reduced pressure, generation of, for example, no gases takes place.
An object of the invention is to provide a silicon-containing copolymer composition and a partially cross-linked, solvent-soluble, silicon-containing copolymer formed from the composition which solve the above-described problems, that is, which can be cured at a lower temperature, which possess a high transparency even when cured in the atmosphere, which have a heat resistance against heating to 400° C. or higher, particularly under reduced pressure, and which can form a film or a coat showing a high chemical resistance and a low relative dielectric constant.
Another object of the invention is to provide a partially cross-linked, silicon-containing copolymer which has the above-described properties and which allows to form a film or a coat having a more thickness by one coating and thus having an excellent mechanical strength and an excellent flexibility, and to provide a silicon-containing copolymer composition to be used for forming the partially cross-linked silicon-containing polymer.
A further object of the invention is to provide a method for curing the above-described composition or the above-described solvent-soluble, cross-linked, silicon-containing copolymer, to provide a film or a coat having an excellent heat resistance, an excellent chemical resistance, an excellent transparency and a low dielectric constant and to provide a display such as a plasma display panel (PDP) using a cross-linked, silicon-containing copolymer including such film or coat as a dielectric layer, a partition (rib layer) and/or a vacuum-sealing agent or a liquid crystal display (LCD) using these as an interlayer dielectric film/and or an oriented film, or a semiconductor device having the cross-linked, silicon-containing copolymer as a layer-constituting member.